In an optical communication system for trunk lines, large-capacity optical transmission is executed by using a wavelength multiplexing communication system in which multiple optical signals having different wavelengths are transmitted through a single fiber. In such a wavelength multiplexing communication system, variable wavelength lasers have become indispensable devices because the oscillation wavelength can be changed in a wide wavelength range. In wavelength multiplexing communication, predetermined wavelength channels (ITU-T grids) are set in advance, and the wavelengths are controlled in variable wavelength lasers to be adjusted to the corresponding grids, respectively.
The transmission capacity of a wavelength multiplexing communication system is represented by a product of the bit rate per wavelength channel multiplied by the number of wavelength channels to be used, and hence, the greater the number of wavelength channels becomes, the more the transmission capacity increases. The number of wavelength channels is determined by a range of wavelengths to be used (for example, a range of wavelengths from 1525 nm to 1565 nm, called “C-band”), and the wavelength interval of the wavelength channels. Therefore, given a certain wavelength range, if the wavelength interval can be set narrower, the number of wavelength channels can be greater, and hence, the transmission capacity becomes greater.
In a current wavelength multiplexing communication system, the modulation baud rates of wavelengths are 10 Gbaud and 25 Gbaud, and the wavelength interval is set to 50 GHz (about 0.4 nm) as illustrated in FIG. 1A. In contrast to this, for the next-generation wavelength multiplexing communication system, increasing the transmission capacity has been under investigation that uses a Nyquist method or an optical orthogonal frequency division multiplexing (optical OFDM) method, so that the wavelength interval is narrowed down to a modulation baud rate being the physical limit. Specifically, as illustrated in FIG. 1B, it has been investigated to narrow the wavelength interval down to a modulation baud rate being the physical limit, for example, the wavelength interval 25 GHz for 25 Gbaud, for increasing the transmission capacity.